Surface inspection with variable digital filtering

ABSTRACT

A semiconductor wafer, which is an inspection object, is stuck by vacuum on a chuck and this chuck is mounted on an inspection object movement stage consisting of a rotational stage and a translational stage, located on a Z-stage. The rotational stage provides a rotational movement and the translational stage provides a translational movement. And when a foreign particle or a defect on an inspection object surface is detected, the parameter of digital filtering is dynamically changed during inspection, and the foreign particle or the defect is differentiated using the result after removing a low frequency fluctuation component to be a noise component.

BACKGROUND OF THE INVENTION

The present invention relates to a method for inspecting a foreignparticle or a defect and the like present on a surface of an inspectionobject, and an apparatus for inspecting the foreign particle, the defector the like.

In a surface inspection, for example, a circuit is formed by patterntransfer on a bare-wafer and following etching, in a manufacturingprocess for a semiconductor device. In various manufacturing processesof a semiconductor device for forming a circuit, a foreign particle or adefect and the like attached on a bare-wafer surface accounts for asignificant factor of lowering a yield. A foreign particle or a defectattached on a wafer surface is controlled in each step of manufacturingprocesses, and an apparatus to detect a foreign particle attached on abare-wafer surface or a defect present on a wafer surface and the likewith high sensitivity and high throughput, is a wafer surface inspectionapparatus.

Methods to detect a foreign particle or a defect on a wafer surface aremainly classified into methods using a charged particle beam of anelectron beam or the like and methods using a light beam, and themethods using a light beam include a method to take an image of a wafersurface by using a camera and analyze image information and a method todetect a scattered light on a wafer surface by a light receiving elementlike a photoelectron multiplier tube and analyze the extent of lightscattering. The latter includes JP-A-63-143830.

In a surface detection apparatus of a method in which a laser beam isirradiated on a wafer, generally a laser beam is irradiated on a wafersurface and a scattered light generated from a foreign particle by theirradiation, is detected by a detector and A/D-converted, and then thesize of a foreign particle/defect is calculated from digital dataobtained after A/D conversion. To attain high throughput of inspection,a method is adopted that an inspection table, where a work (a wafer) ismounted, is rotated at high speed, and a stage where the inspectiontable is horizontally mounted in uniaxial direction, is scanned. Basedon size information on a foreign particle/defect and coordinateinformation from the stage, a foreign particle/defect map on an entiresurface of the work is calculated.

Aside from a signal generated from a foreign particle/defect of adetection object, reflected light from an inspection object includes alow frequency fluctuation component depending on surface condition, filmtype or film thickness, and surface roughness. In addition, influencedby vibration or the like from an inspection object movement unit, a lowfrequency component is generated. This low frequency component is notconstant, because it is decided by parameters consisting of a size ofillumination light, a speed of the inspection object movement unit andmovement position.

Conventionally, though the frequency component was removed or controlledby an analogue filter, since it is difficult for a Cut-off frequencysetting to be flexibly varied as being determined by a circuit constant,it was difficult to respond to each of the conditions described above.

In addition, considering distortion of a passing signal, since a passingsignal band is required to have a margin, it was hard to have anattenuation frequency band sufficiently wide, and it was difficult forthe low frequency component to be removed or controlled accurately.

Accordingly, since a detection determination threshold level has to beraised by the degree of the low frequency fluctuation componentremaining after passing the analogue filter, there remained a problemthat detection sensitivity was deteriorated by this degree.

In addition, depending on surface condition, film type or filmthickness, and surface roughness, reflected light generated from aforeign particle/defect varies itself, therefore there was a problemthat it was difficult to respond only with a fixed threshold.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a surface inspectionapparatus and a surface inspection method enabling to respond to a widevariety of wafers with a different kind of film type, film thickness,surface roughness and crystal orientation or the like.

In addition, another object of the present invention is to provide asurface inspection apparatus and a surface inspection method enabling toreduce the influence received from an inspection object movement unit ofthe inspection apparatus itself, and enhance the accuracy of detectiondetermination.

According to one embodiment of the present invention, in digitalfiltering of a signal obtained by a surface inspection, at least oneparameter for the digital filtering is dynamically varied duringinspection.

According to another embodiment of the present invention, in digitalfiltering of a signal obtained by a surface inspection, at least oneparameter for the digital filtering is controlled in response to aradius of an inspection object.

According to still another embodiment of the present invention, asurface inspection apparatus for detecting a foreign particle or adefect present on an inspection object surface or inside the proximityof the surface is configured so as to detect a foreign particle/defectby using results obtained by a removing treatment of an undesired lowfrequency fluctuation component.

It is desirable that the removing treatment of the undesired lowfrequency fluctuation component be accomplished by a digital filteringtreatment of plurality of digital data obtained from a signal receivedat the surface inspection.

It is desirable that the digital filtering treatment be a frequency bandlimitation filtering treatment which removes the undesired low frequencyfluctuation component.

It is desirable that the frequency band limitation filtering treatmentbe a low-pass filtering treatment which extracts the undesired lowfrequency fluctuation component and be configured so as to subtract theresult of the low-pass filtering treatment from the digital data.

It is desirable that the frequency band limitation filtering treatmentbe a high-pass filtering treatment or a band-pass filtering treatmentwhich removes the undesired low frequency fluctuation component.

In addition, it is desirable that the Cut-off frequency of the digitalfiltering treatment be variable.

In addition, it is desirable that the Cut-off frequency of the frequencyband limitation filtering treatment be determined based on any one or acombination of plurality of (1) a main scanning rotational speed of aninspection object movement stage, (2) coordinate position in thesub-scanning direction obtained by the above coordinate detection unit(3) a size of an illumination spot, and be set every time when acondition varies.

In addition, when the above particle diameter calculation devicedetermines the Cut-off frequency of the frequency band limitationfiltering treatment, it is desirable to use further any one or acombination of plurality of (1) type or thickness of film formed (2)surface roughness (3) crystal orientation (4) warpage amount, on aninspection object surface.

According to the present invention, even in the case of differentsurface conditions of an inspection object, as it is possible to reducethe low frequency component of the reflected light, it is not necessaryto raise a detection determination threshold level, therefore a weaksignal can be detected and an inspection object can be accuratelymeasured.

Alternatively, because reduction of the low frequency component of thereflected light is enabled in each condition of a size of illuminationlight, a speed of an inspection object movement unit or movementposition, a proper threshold can be set.

Alternatively, even in the case that surface condition of an inspectionobject differs depending on film type or film thickness, surfaceroughness, inspection can be executed with the same performance as inthe case of a standard wafer.

Alternatively, because of no signal distortion caused by analoguefiltering, a passing signal has no distortion and it is possible tonarrow passing band width and further to obtain symmetric attenuationproperty in both bands, therefore, an accurate inspection can beaccomplished.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic configuration according to one embodiment ofthe present invention.

FIG. 2A shows a side view of an illumination spot according to oneembodiment of the present invention.

FIG. 2B shows a plan view of an illumination spot according to oneembodiment of the present invention.

FIG. 3 shows scanning according to one embodiment of the presentinvention.

FIG. 4 shows the difference (the signal width difference) of a foreignparticle/defect signal caused by the scanning position differenceaccording to one embodiment of the present invention

FIG. 5 shows a foreign particle/defect signal and a threshold accordingto one embodiment of the present invention.

DESCRIPTION OF THE INVENTION

Although the following description will be made below on embodiments ofthe present invention using the accompanying drawings, the apparatus andthe method according to the present invention are not limited to theconfigurations shown in each drawing and various changes andmodifications may be made within the spirit of the invention.

FIG. 1 shows first embodiment of a foreign particle/defect inspectionapparatus using a foreign particle/defect detection method according tothe present invention. The semiconductor wafer 100, which is aninspection object, is stuck fast to the chuck 101 by vacuum and thischuck 101 is mounted on the Z-stage 105 via the inspection objectmovement stage 102 configured with a rotational movement unit, which iscapable of scanning with a nearly constant rotational angle speed,consisting of the rotational stage 103, and a translational movementunit consisting of the translational stage 104. The rotational movementθ is carried out on the rotational stage 103 and the translationalmovement r is carried out on the translational stage 104.

FIGS. 2A and 2B are a plan view and a side view showing theillumination/detection optical system 110 located above thesemiconductor wafer 100. An illumination unit according to the presentembodiment uses a laser light source as the light source 200 of anillumination light. The irradiating light 201 consisting of a laserlight shot from the light source 200 enters into the irradiating lens202 and the illumination spot 203 with a predetermined size is formed.The irradiating light 201 is, for example, P polarization, and isconfigured so as to enter obliquely with approximately Brewster's angleto crystal Si, on a surface of the semiconductor wafer 100, which is aninspection object. Therefore, the illumination spot 203 is substantiallyoval-shaped, and the inside area of a contour line where illuminancecomes down to 1/e² (e: base of natural logarithm) of that at the centerof the illumination spot 203, is defined anew as the illumination spot.

The width 204 in the long axis direction of the illumination spot 203 isdescribed as d1 and the width 209 in the short axis direction of thesame is described as d2. With the illumination spot 203, θ-scanning 208is executed as indicated by the dotted arrow in FIG. 2B.

As shown in FIG. 3, by changing and combining with time the rotationalmovement 8 and the translational movement r, the inspection objectmovement stage 102 causes relatively the illumination spot 203 to scanspirally on the approximately whole surface of the semiconductor wafer100. While the rotation stage rotates one turn, scanning moves as muchas the distance Δr. When Δr>d1, because an illumination light is notradiated on the semiconductor wafer 100 in spiral scanning and gap areanot to be inspected is formed, normally a setting of Δr<d1 is used.Although, in the present embodiment, scanning of the illumination spot203 is executed from the inner periphery toward the outer periphery ofthe semiconductor wafer 100, the reverse scanning direction isacceptable.

In addition, in the embodiment, in the approximately whole area from theinner periphery toward the outer periphery of the semiconductor wafer100, the rotational stage 103 is driven at a approximately constantangular speed and the translational stage 104 is driven nearly at aconstant linear speed. FIG. 4 shows, as a result of the above, therelative movement linear speed of the illumination spot 203 to thesurface of the semiconductor wafer 100, becomes larger at the outerperiphery area compared with that at the inner periphery area.

On the inspection object movement stage 102, the inspection coordinatedetection device 106 is provided in order to detect the main-scanningcoordinate position θ and the sub-scanning coordinate position r.Although, in the present embodiment, as a means of obtaining positioninformation, a rotary encoder (primary position acquisition section) ofan optical scanning type is used to detect the main-scanning coordinateposition θ, and a linear encoder (secondary position acquisitionsection) of an optical scanning type is used to detect the sub-scanningcoordinate position r, instead of both encoders, other type detectorswith a different detection principle may be used as long as a sensorcapable of detecting an angle or a position on a straight line with highaccuracy is used.

The collecting lens 205 is configured so as to collect a scattered lightwith a low elevation angle in order to collect efficiently the scatteredlight even from a minute foreign particle which follows Rayleighscattering. In this configuration, the scattered light from the foreignparticle/defect 206 passes the collecting lens 205 and is detected by alight detection unit consisting of the light detector 207. The lightdetector 207, which detected the scattered light, converts the scatteredlight to an electric signal (an inspection signal) and outputs as ascattered light signal. Although, in the present embodiment, aphotoelectron multiplier tube is used as the light detector 207, otherlight detectors with different detection principles may be used as longas the light detector can detect a scattered light from a foreignparticle with high sensitivity.

As described above, in the present embodiment, in the approximatelywhole area from the inner periphery toward the outer periphery of thesemiconductor wafer 100, the rotational stage 103 is driven at anapproximately constant angular speed, and the relative movement linearspeed of the illumination spot 203 to the surface of the semiconductorwafer 100 becomes bigger at the outer periphery compared with that atthe inner periphery. Therefore, a time period while a foreign particleon the semiconductor wafer 100 crosses the short axis 209 and the widthd2 of the illumination spot 203, is smaller in the case where theforeign particle is present in an outer periphery area of thesemiconductor wafer 100 compared with that in the case where present inan inner periphery area. Therefore, as shown in FIG. 4, time-varyingwave form of a signal intensity of the scattered light signal obtainedby the light detector 207 via the amplifier 111 has generally thesmaller half bandwidth of signal peak, in the outer periphery area, thatis, in the case where the foreign particle is present in the largerradius area of the scanning direction.

Then, a signal treatment according to the present embodiment will beexplained below. After the scattered light signal, which is converted toan electric signal (an inspection signal) by the light detector 207, isamplified by the amplifier 111, the scattered light signal is sampledevery sampling time interval ΔT predetermined by A/D conversion unitconsisting of A/D converter 112, and converted to digital data. Thesampling time interval ΔT is determined so as to be able to sample thesignal wave form shown in FIG. 4 with sufficient time resolution. Forexample, when the half bandwidth in the most outer periphery area havingthe minimum signal wave form width as shown in FIG. 4, is described asΔSout, ΔT is determined as ΔT=ΔSout/10. By this sampling, a group oftime series digital data corresponding to the signal wave form shown inFIG. 4. are obtained.

Incidentally, the group of time series digital data includes the signalcomponent 500 of low frequency as shown in FIG. 5 in addition tointensity information of the scattered light corresponding to a size ofthe detected foreign particle/defect, which is essentially needed.Generally, the signal component of low frequency does not become a fixedvalue because it varies depending on a main scanning rotational speed ofthe inspection object movement stage, coordinate information in thescanning direction obtained by the coordinate detection unit, the sizeof the illumination spot, and further, type or thickness of film formed,surface roughness, crystal orientation and warpage amount on theinspection object surface. Therefore, in order to calculate correctly asize of the foreign particle/defect, it is necessary to remove theinfluence from the low frequency component.

Consequently in the present embodiment, toward the digital data from theA/D converter 112, the data of only the low frequency component isgenerated by the variable low-pass filter 114 treatment as one exampleof digital filtering, followed by subtraction by the subtracter 115 fromdata obtained by the A/D converter 112 (digital filtering section) andonly the intensity information of the scattered light corresponding tothe size of the foreign particle/defect is extracted.

Here, the Cut-off frequency which is one example of a parameter of thevariable low-pass filter 114, is dynamically controlled by thecalculator 116 (parameter changing section), based on the informationincluding rotational speed of an inspection object movement stage,coordinate position in the scanning direction obtained by the coordinatedetection unit, size of the illumination spot, further, type orthickness of film formed, surface roughness, crystal orientation andwarpage amount on the inspection object surface. The calculationparameter of this calculator is based on information from the inspectioncoordinate detection device 106 and the upper CPU 107.

Cut-off frequency=1/(short radius of illumination spot/rotationalspeed/(2*circle ratio*radius coordinate position))/A

“A” is specified from film type, film thickness, surface roughness,crystal orientation and warpage amount.

Film type, film thickness, surface roughness, crystal orientation andwarpage amount are set by a user via an input unit not shown, andcalculated in an inspection apparatus. As this input unit, a pointingdevice such as a keyboard or a mouse may be used. In addition, anindependent memory in which necessary information described above isstored, may be input into the inspection apparatus via an interface notshown.

The scattered light intensity value obtained as a result of the dataprocessing, is compared with the predetermined detection threshold inthe foreign particle/defect determination device 108, and the foreignparticle/defect determination device 108 generates foreignparticle/defect determination information if the scattered lightintensity value is not less than the threshold. When the foreignparticle/defect determination information is generated, the foreignparticle/defect coordinate detection device 109 calculates thecoordinate position of the detected foreign particle/defect based oninformation from the inspection coordinate detection device 106. Inaddition, the particle diameter calculation device 117 calculates thesize of the foreign particle/defect detected from the scattered lightintensity value.

In this manner, in the present embodiment, toward the signal obtained bythe amplifier 111, after removing the influence of the low frequencycomponent by the variable low-pass filtering treatment, the size of theforeign particle/defect is calculated. By execution of parameterchanging treatment where parameter of this digital filtering is varieddynamically during inspection, as shown in FIG. 5, even in the case ofsuperposition 501 of the foreign particle/defect signal on the lowfrequency component, the low frequency component removal 502 causes theforeign particle/defect signal to be the detection signal 506 after thepresent embodiment, and since the offset by the extent of the signalcomponent 500 of the low frequency wave is not necessary like theconventional threshold 504 and can be used as the threshold 505 afterthe present embodiment, the foreign particle/defect signal 503 which hasnot been detected conventionally, can be properly detected.

Although, in the present embodiment, after the undesired low frequencyfluctuation component is extracted by the low-pass filtering treatment,the subtraction thereof from the original data provides the removal ofthe low frequency fluctuation component, the configuration is obviouslyaccepted where a high-pass filtering treatment or a band pass filteringtreatment in which the low frequency fluctuation component is removeddirectly.

The result of the surface inspection is output from an output unit notshown. As the output unit, a printing unit such as a display unit or aprinter and the like, may be used. In addition, result information maybe stored in a memory built in the inspection apparatus via an interfacenot shown. Alternatively, the result information may be stored in anindependent memory via an interface not shown.

It should be noted that, according to the present embodiment, althoughforeign particles/defects on the inspection surface are differentiatedby the electric signal (inspection signal) based on the scattered light,it is not limited to this method, furthermore, by providing a lightdetecting unit where a diffracted light or a reflected light, which isemitted by the irradiating light from the inspection object, is detectedand converted to an electric signal (inspection signal), differentiationof foreign particles/defects can be accomplished by the electric signal(inspection signal).

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A surface inspection apparatus for detecting a foreign particle or adefect present on an inspection object surface or inside the proximityof the surface, comprising: an inspection object movement stage having arotational movement unit and a translational movement unit configured soas to be able to scan said inspection object at an approximatelyconstant rotational angular speed; a laser beam source; an illuminationunit irradiating an laser beam emitted from said laser beam source on anillumination spot of a predetermined size on the inspection objectsurface; a scattered/diffracted/reflected light detection unit, whereinan irradiating light detects a scattered/diffracted/reflected light atsaid illumination spot and converts to an electric signal; an A/Dconverter for conversion of said electric signal into digital data; aparticle diameter calculation device for calculating a size of theforeign particle or the defect from the digital data obtained by saidA/D conversion; wherein said A/D conversion unit executes sampling ofsaid electric signal at approximately constant sampling time intervals,and said particle diameter calculation device executes detection of theforeign particle/defect by using a result of a removal treatment of anundesired low frequency fluctuation component on the digital dataobtained by said A/D converter. 2-18. (canceled)